The air we breathe is made up of the gases mainly of nitrogen, oxygen and carbon dioxide. In this section we will discuss how oxygen, carbon dioxide and carbon monoxide can become poisonous to us.



Oxygen is essential to metabolism. Ironically, excessive amounts of oxygen or excessively high oxygen partial pressures can be detrimental or even fatal. There are two categories of this condition; chronic and acute oxygen toxicity. 

Chronic toxic effects are manifested after the diver is exposed to oxygen greater than 0.6 bar over a long period of time. Toxicity is cause through the production of molecular oxygen with free radicals. These free radicals are highly reactive and react with proteins and tissues, particularly of the alveoli in the lungs thus destroying them. This will cause coughing and difficulties in breathing. On continued exposure, the alveoli will start to swell and fill up the lung sacs with fluid, depriving it of oxygen causing permanent damage to the lungs and death may occur.

Acute toxic effects affect the central nervous system. It occurs when the person is exposed to partial pressure of 1.6bar or higher. Symptoms show within a few minutes of exposure. Warning signs are face and lips twitching. Soon after dizziness, nausea and abnormal breathing patterns. After which, he will become disorientated, lose consciousness, have convulsions and finally drown.


Carbon dioxide builds up in the body as waste products of metabolism. A raised partial pressure of carbon dioxide, called hypercapnia, when high enough causes carbon dioxide toxicity. This can ultimately cause the person to become hypoxic. 

The build up carbon dioxide can be the result of several factors. One is the air which the diver is breathing from could be contaminated. Two, the diver deliberately under ventilate to conserve air. Lastly, the diver is exerting himself in the case where he has to fight strong currents. 

As carbon dioxide builds up, the diver would experience throbbing headaches and find himself breathing harder. Dizziness might set in. If carbon dioxide concentration continues to build up, there would be a reduction in pulse rate and blood pressure. Finally he will lose consciousness and drown.


Carbon monoxide is highly toxic because it has a very high affinity to haemoglobin. About 250 times more then oxygen. If a person breathes gas contaminated with carbon monoxide, the level of carboxyhaemoglobin increase the longer he breathes the air. As a result of this, a diver breathing a tank contaminated will too high a concentration of carbon monoxide will have less haemoglobin carrying oxygen to the rest of his body, causing tissues to become hypoxic. 

Initially he will develop headaches, dizziness and breathlessness. Soon he becomes exhausted and confusion sets in. Following this, unconsciousness and death. Carboxyhaemoglobin is cherry red so a person with carbon monoxide hypoxia has flushed cheeks and lips.


Impurities can get into the tank the diver uses. Oil, in sufficient quantities, damages the lungs and causing pneumonia symptoms. Oxides of nitrogen are acidic and can irritate and damage the lungs.



DCS Nitrogen Narcosis Gas toxicity





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